/**
  * Retrieve registered license option keys via `apppresser_license_keys_to_check` filter
  * @since  1.0.1
  * @return array  All license option keys
  */
 public static function license_keys()
 {
     if (empty(self::$license_keys)) {
         $appp_settings = self::run();
         self::$license_keys = apply_filters('apppresser_license_keys_to_check', self::$license_keys, $appp_settings);
     }
     return self::$license_keys;
 }